Firefox itself needs large market share, as a bulkhead against sites that work only in {IE6|Chrome}.

Mobile OSes need large enough market share because you have to persuade the proprietors of important services to make their services available on the OS — if those services aren't available, your OS won't have enough utility for most users.

But it is possible to get a large number of services working on an unpopular mobile OS — if there's a common standard that unpopular platforms can make use of, which is good enough for most users — such as web apps.

(For example, DuckDuckGo can succeed or fail on merit, because it's a viable alternative. In the Linux bubble, most apps don't specifically target unpopular distros, but they can make use of the same source code or the same packaging formats.)

So if we want better mobile OSes, we should support interoperability of apps.

To be fair people said the same thing thing to Apple when they decided to enter the phone market

Today the technology is mature enough, so it's more about the (small) gimmicks like fingerprint scanners.